my response would be, "why do you want to know?"

if they want to know because they like it (or just are interested) and want to buy one (whatever it is, bike or otherwise) fine, we can discuss the item and its merits, cost, etc. i would expect this would normally be a bike person and they would have some idea of bike prices. if they just want to talk about money and how much i do or don't have, well, that's not really something i'm comfortable discussing with strangers. you can usually tell when people are asking for this reason, but asking their motivation never hurts.

lately there has been so much bike theft that if a total stranger asked me that i would wonder if they were trying to case out whether they should steal my bike!